Field of the Invention
The present invention relates to an optical filtering device made from an optical fibre, in particular an optical fibre filter which can be used in a system for the transmission of wavelength division multiplexing (abbreviated “WDM”) optical signals. The present invention also relates to a process for the manufacture of this filter, to an optical fibre which can be used to form this filter, to a system for the transmission of WDM signals using this filter and to a method for filtering optical signals.
Description of the Related Art
In detail, a WDM optical signal is a (digital or analogic) signal comprising a plurality of N optical signals which are independent of each other and each of which has a respective transmission wavelength λ1, λ2 . . . λN different from that of the other signals. Each transmission wavelength defines a transmission “channel”. Moreover, each signal has, associated with it, a respective wavelength bandwidth Δλ of predefined size—called a “channel bandwidth” or “channel (spectral) size”—which is centred on the corresponding transmission wavelength. The channel size depends, typically, on the characteristics of the laser sources used and on the type of modulation used in order to associate the information to be transmitted with the signal. In the absence of modulation, typical spectral amplitude values of a signal emitted by a laser source are in the region of 10 MHz, while in the case of modulation outside the range of 2.5 Gbit/s they are in the region of 5 GHz.
The WDM signal also has a “spacing between channels” defined as the wavelength (or—in an equivalent manner—frequency) separation between the central wavelengths of two adjacent channels. In order to transmit to a high number of channels in one of the so-called “transmission windows” of the optical fibres and in a useful amplification bandwidth of the optical amplifiers, the spacing between the channels of a WDM signal is, typically, in the region of one nanometre.
Generally, in a WDM system, the transmission of signals occurs in the following manner: the various signals are first generated by respective optical sources, then multiplexed so as to form a WDM signal, next transmitted along the same optical fibre transmission line and, finally, demodulated so as to be received by respective receivers.
In recent wavelength multiplexing optical amplification and transmission systems (able to transmit, along the same fibre, a very high number of channels—for example 128—distributed over a particularly wide spectral bandwidth—for example 70 nm) and, more generally, in optical signal processing apparatus, for both instrumentation and sensors, devices made entirely of optical fibre, without any propagation of the light in free space, are being increasingly used. In particular, these devices are required for the operations of spectral filtering, multiplexing and demultiplexing of the channels and separation of the transmission spectrum into bands.
With regard to spectral filtering, it is necessary to use both devices with a high wavelength selectivity for filtering of individual channels and wider-bandwidth devices for equalization of the channels in the amplification bandwidth of the optical amplifiers. Equalization of the channels is necessary since the gain spectrum of erbium-doped optical fibre amplifiers (which constitute the most widely used optical amplification means) has a significantly unequalized form in the region between 1530 and 1560 nm. Despite the progress achieved in the development of glass matrices for silica-based optical fibres containing various co-dopants able to “flatten” the spectral gain curve, at the moment silica-based fibres which have a sufficiently uniform gain profile such as not to require external equalization are not available.
The configuration most used to form wideband optical gain modules involves the use of an equalizer filter arranged between two active-fibre amplification stages. The insertion of the filter between the two amplification stages has the fundamental advantage of allowing a spectral “redistribution” of the power available for amplification, instead of simple suppression of the power in the wavelength regions with a higher gain. The spectral profile of the filter which offers maximum equalization depends on the operating conditions of the amplifier (and, therefore, on the power of the pump radiation supplied to each stage) as well as the number and the wavelength distribution of the channels. In recent systems where there is the possibility of channel addition/extraction, the number and the distribution of the channels may change depending on the configuration chosen by the system manager.
For the abovementioned reasons it has become important to have optical filters which can be efficiently integrated with the active fibre, with low insertion losses and with a spectral profile which can be easily modified depending on the specific use of the individual amplifier.
Different types of filters made directly using optical fibre are known.
A first type of filter is distinguished by the fact that the fibre has a portion with a sudden variation in diameter, i.e. a tapered portion. This region induces, in each signal passing through it, an attenuation which depends on the wavelength of the said signal. In this way, therefore, spectral filtering is performed. The spectral form of the attenuation of these filters is substantially sinusoidal on the wavelength.
Another type of filter, which is known as a “Fabry-Perot” filter, is formed by an optical fibre and two Bragg gratings formed in the fibre itself and operating as mirrors so as to define an optical resonator.
More recently so-called long period gratings (LPG) have been developed, said gratings being distinguished by periodic variations in the index profile of a fibre (typically by means of exposure to UV radiation) and also allowing wavelength filtering to be performed.
A further class of filters is that defined by an interferometric structure of the Mach-Zehnder type. Such a structure must be able to perform separation of an optical signal into two different distributions of electromagnetic field, propagate these distributions along respective optical paths into which it is possible to introduce, in a controlled manner, a mutual delay and, subsequently, combine again the two electromagnetic field distributions so as to obtain an optical interference signal, the intensity of which is a function of the wavelength.
FIG. 1 shows schematically a Mach-Zehnder filter 50 of known type, able to operate with two distinct field distributions. This interferometric structure comprises a first and a second optical fibre 51, 52 joined at two different points by means of a first and a second fusion coupler 53, 54, for example of the 50/50 (or 3 dB) type. The filter 50 is able to receive at its input a signal Sin from a first end of the first fibre 51 and provide at its output a filtered signal Sout to a second end of the first fibre 51. In the section between the couplers 53, 54, the fibres 51, 52 define optical paths of different length. The difference in optical path length between the two fibres 51, 52 may be due to the fact that they have different transmissive properties, so that the signals which are propagated in one fibre have a different speed from those which are propagated in the other fibre or, as shown in the figure, may be due to the fact that they have different lengths L and L+ΔL in the section considered.
The couplers 53, 54 allow power coupling between the electromagnetic fields which are propagated in the two fibres 51, 52. In particular, the function of the first coupler 53 is that of exciting two different electromagnetic field distributions in the optical fibres 51, 52 from the signal Sin. These electromagnetic fields, which are propagated along different optical paths, accumulate a relative phase difference Δφ which is not zero and defined by:
                              Δϕ          ⁡                      (            λ            )                          =                              2            ⁢                          π              ·                              n                eff                            ·              Δ                        ⁢                                                  ⁢            L                    λ                                    (        1        )            
where neff is the effective refractive index of the mode which is propagated in the fibres, λ is the wavelength and ΔL is the difference in length between the sections of the two fibres 51, 52 comprised between the two couplers 53, 54.
The second coupler 54 is designed to combine again the two electromagnetic fields, generating an interference between them which may be constructive or destructive, depending on the phase shift Δφ accumulated.
In the simplest case where the fibres 51, 52 are identical and the couplers 53, 54 have an optical power dividing ratio equal to 50/50 (3 dB couplers), the optical powers at the two outputs of the second coupler 54, indicated respectively by P1 and P2, are defined by the following equations:
                                                                        P                1                            =                                                cos                  2                                ⁡                                  (                                                                                    Π                        ·                                                  n                          eff                                                ·                        Δ                                            ⁢                                                                                          ⁢                      L                                        λ                                    )                                                                                                                        P                2                            =                                                sin                  2                                ⁡                                  (                                                                                    Π                        ·                                                  n                          eff                                                ·                        Δ                                            ⁢                                                                                          ⁢                      L                                        λ                                    )                                                                                        (        2        )            
FIG. 2 shows the normalised transmission spectrum T(λ) of the filter 50 at the output of its first fibre 51, in the case where ΔL is equal to 5 μm and neff is equal to 1.462. The period of this curve is not constant and is a function of the characteristics of the waveguides used. Having a different response for the different wavelengths, the interferometer may be advantageously used as an optical filter.
A Mach-Zehnder filter such as that described above is, however, difficult to use in practice, owing to its extreme sensitivity to external disturbances (for example variations in temperature) and variations in form (in particular variations in curvature of the fibre). These phenomena cause variations in the effective refractive index neff and, therefore, in the optical path, which are generally different for the two fibres. The behaviour of this device, which is ideally described by the equations (1) and (2), therefore cannot be predicted precisely in a real situation.
In order to overcome this drawback, a solution which combines the two waveguides in a single compact structure has been proposed. The U.S. Pat. No. 5,295,205 in the name of Corning proposes a filter formed by introducing two optical fibres which are different from each other inside a glass tube, collapsing the tube onto the fibres after creating a vacuum inside the tube and, finally, heating and stretching the tube in two regions located at a distance from each other so as to form two tapered regions which define modal couplers. The fibres also have different propagation constants in the zone lying between the two couplers, resulting in a relative delay between the optical signals propagated therein.
The Applicant considers that this solution is difficult to realise on account of the technological complexity of certain steps in the production process, in particular the operations for collapsing the glass tube around the fibres after creating a vacuum in the tube and forming the couplers at a distance from each other determined on the basis of the desired spectral form and independently of the geometry of the tapered region.
An alternative method of producing a Mach-Zehnder interferometer is that described in international patent application WO00/00860 in the name of Corning. This document describes a coaxial optical device comprising an optical fibre and a coupling regulator integral with the optical fibre. The optical fibre is single-mode in the third spectral window of optical telecommunications and a glass tube with a refractive index lower than that of the cladding is collapsed onto the fibre, as described in the already mentioned U.S. Pat. No. 5,295,205. In the region where the collapsed tube is present, the refractive index profile is modified so as to allow locally the transmission of two modes, in particular the modes LP01 and LP02. These modes, which are mutually perpendicular by definition, define two distinct field distributions which, as they are propagated, accumulate a relative phase difference Δφ. In the region occupied by the glass tube, non-adiabatic tapered zones able to induce power coupling between the modes are formed. The tapered zones are formed by means of the normal technique for manufacturing fusion couplers, by causing sudden reductions in the diameter of the fibre and the tube collapsed onto it, such as to obtain coupling between the symmetrical modes LP01 and LP02, but avoid coupling with the mode LP03.
The Applicant also notes that the device described above requires the execution of technologically complex manufacturing steps, such as collapsing of a glass tube, under vacuum, onto an optical fibre and the formation of non-adiabatic tapered zones such as to have a high value of the coupling factor between the symmetrical modes LP01 and LP02, but without exciting other higher symmetrical modes such as the mode LP03 (where “coupling factor” or “splitting ratio” is understood in this case as being the ratio between the power transferred to the mode LP02 and the remaining power in the mode LP01).
The Applicant therefore notes that the Mach-Zehnder optical fibre filters of the known type are made using complex technology which does not allow easy control of the filter parameters. The critical nature of the manufacturing process therefore results in high costs and fairly low production outputs.